Cassidy implied his second-round opponent gets preferential treatment from the officiating crew.
"I think they sell a narrative over there that it's more like the New York Saints, not the New York Islanders. They play hard, they play the right way, but I feel we're the same way. And the exact calls that are getting called on us do not get called on them, and I don't know why," Cassidy said after the loss.
"Maybe we need to sell them more, flop, but that's not us," he added. "You'd just hope they see them. I mean the same calls go against us. It's not like I'm sitting there going 'well every call against us sucks,' it's not true.
"... They need to be penalized on those plays. They've done a great job selling that narrative that they're clean. ... They commit as many infractions as we do, trust me. It's just a matter of calling them. That's the part that I guess gets frustrating but you play through it."
Boston was penalized four times in the contest, and New York made the most of its opportunities while converting three power-play tallies to take a 3-2 series lead. The Bruins were given two-man advantages in the game.
After taking the hit to his wallet, Cassidy stated he believes what he said was fair.
"We're told before we have to keep our comments civil. I thought it was. They didn't see it that way," the coach said Tuesday, per The Athletic's Fluto Shinzawa.
Islanders head coach Barry Trotz was also critical of the officiating after Game 4.
Game 6 is slated for Wednesday in New York.